1. Field of the Invention
This invention relates to an apparatus for molding thixotropic materials into articles of manufacture. More specifically, the present invention relates to a thermally efficient and thermally shock resistant apparatus for molding thixotropic materials into articles of manufacture.
2. Description of the Prior Art
Metal compositions having dendritic structures at ambient temperatures conventionally have been melted and then subjected to high pressure die casting procedures. These conventional die casting procedures are limited in that they suffer from porosity, melt loss, contamination, excessive scrap, high energy consumption, lengthy duty cycles, limited die life, and restricted die configurations. Furthermore, conventional processing promotes formation of a variety of microstructural defects, such as porosity, that require subsequent, secondary processing of the articles and also result in use of conservative engineering designs with respect to mechanical properties.
Processes are known for forming these metal compositions such that their micro-structures, when in the semi-solid state, consist of rounded or spherical, degenerate dendritic particles surrounded by a continuous liquid phase. This is opposed to the classical equilibrium microstructure of dendrites surrounded by a continuous liquid phase. These new structures exhibit non-Newtonian viscosity, an inverse relationship between viscosity and rate of shear, and the materials themselves are known as thixotropic materials.
One process for forming thixotropic materials requires the heating of the metal composition or alloy to a temperature which is above its liquidus temperature and then subjecting the liquid metal alloy to a high shear rate as it is cooled into the region of two phase equilibria. A result of the agitation during cooling is that the initially solidified phases of the alloy nucleate and grow as rounded primary particles (as opposed to interconnected dendritic particles). These primary solids are comprised of discrete, degenerate dendritic spherules and are surrounded by a matrix of an unsolidified portion of the liquid metal or alloy.
Another method for forming thixotropic materials involves the heating of the metal composition or alloy (hereafter just "alloy") to a temperature at which most, but not all of the alloy is in a liquid state. The alloy is then transferred to a temperature controlled zone and subjected to shear. The agitation resulting from the shearing action of the material converts any dendritic particles into degenerate dendritic spherules. In this method, it is preferred that when initiating agitation, the semisolid metal contain more liquid phase than solid phase.
An injection molding technique using alloys delivered in an "as cast" state has also been seen. With this technique, the feed material is fed into a reciprocating screw injection unit where it is externally heated and mechanically sheared by the action of a rotating screw. As the material is processed by the screw, it is moved forward within the barrel. The combination of partial melting and simultaneous shearing produces a slurry of the alloy containing discrete degenerate dendritic spherical particles, or in other words, a semisolid state of the material and exhibiting thixotropic properties. The thixotropic slurry is delivered by the screw to an accumulation zone in the barrel which is located between the extruder nozzle and the screw tip. As the slurry is delivered into this accumulation zone, the screw is simultaneously withdrawn in a direction away from the unit's nozzle to control the amount of slurry corresponding to a shot and to limit the pressure build-up between the nozzle and the screw tip. The slurry is prevented from leaking or drooling from the nozzle tip by controlled solidification of a solid metal plug in the nozzle and the plug is formed by controlling the nozzle temperature. Once the appropriate amount of slurry for the production of the article has been accumulated in the accumulation zone, the screw is rapidly driven forward developing sufficient pressure to force the solid metal plug out of the nozzle and into a receiver thereby allowing the slurry to be injected into the die cavity so as to form the desired solid article. The plug in the nozzle provides protection to the slurry from oxidation or the formation of oxide on the interior wall of the nozzle that would otherwise be carried into the finished, molded part. The plug further seals the die cavity on the injection side facilitating the use of vacuum to evacuate the die cavity and further enhance the complexity and quality of parts so molded. The plug further permits a faster cycle time than would otherwise be obtained if a sprue break operational mode was used. The receiver includes a sprue bushing that directs the flow of slurry into the die cavity and also thermally controls the solidification rate of the sprue in order to reduce cycle times and make the machine more efficient.
Currently, the thixotropic molding machines perform all of the heating of the material in the barrel of the machine. Material enters at one section of the barrel while at a "cold" temperature and is then advanced through a series of heating zones where the temperature of the material is rapidly and, at least initially, progressively raised. The heating elements themselves, typically resistance or induction heaters, of the respective zones may or may not be progressively hotter than the preceding heating elements. As a result, a thermal gradient exists both through the thickness of the barrel as well as along the length of the barrel.
Typical barrel constructions of a molding machine for thixotropic materials have seen the barrels formed as long (up to 110 inches) and thick (outside diameters of up to 11 inches with 3-4 inch thick walls) monolithic cylinders. As the size and through-put capacities of these machines have increased, the length and thicknesses of the barrels have correspondingly increased. This has led to increased thermal gradients throughout the barrels and previously unforseen and unanticipated consequences. Additionally, the primary material, wrought alloy 718 (having a limiting composition of: nickel (plus cobalt), 50.00-55.00%; chromium, 17.00-21.00%; iron, bal.; columbium (plus tantalum) 4.75-5.50%; molybdenum, 2.80-3.30%; titanium, 0.65-1.15%; aluminum, 0.20-0.80; cobalt, 1.00 max.; carbon, 0.08 max.; manganese, 0.35 max.; silicon, 0.35 max.; phosphorus, 0.015 max.; sulfur, 0.015 max.; boron, 0.006 max.; copper, 0.30 max. used in constructing these barrels is currently in severe short supply (12 month minimum lead time) and is extremely expensive ($12.00/lb). Two recently constructed 600 ton capacity barrels took one year to procure and cost $150,000 each.
After the lengthy time required for the acquisition of the alloy 718 construction material, the high cost involved in obtaining the construction material, and the time involved in fabricating the barrels themselves, the two 600 ton barrels were put into service molding thixotropic materials, specifically magnesium alloys. Within less than one week of service, approximately 700-900 cycles of the thixotropic molding machines, both of the barrels failed. Upon an analysis of the failed barrels by the present inventors, it was unexpectedly discovered that the barrels failed as a result of thermal stress and more particularly thermal shock in the cold section or end of the barrels. As used herein, the cold section or end of a barrel is that section or end where the material first enters into the barrel. It is in this section that the most intense thermal gradients are seen, particularly in the intermediate temperature region of the cold section, which is located downstream of the feed throat.
During use of a thixotropic material molding machine, the solid state material feed stock, which has been seen in pellet and chip forms, is fed into the barrel while at ambient temperatures, approximately 75.degree. F. Being long and thick, the barrels of these thixotropic material molding machines are, by their very nature, thermally inefficient for heating a material introduced therein. With the influx of "cold" feed stock, the intermediate temperature region of the barrel is significantly cooled on its interior surface. The exterior surface of this region, however, is not substantially affected or cooled by the feed stock because the positioning of the heaters is directly thereabout. A significant thermal gradient, measured across the barrel's thickness, is resultingly induced in this region of the barrel. Likewise, a thermal gradient is also induced along the barrel's length. In this intermediate temperature region of the barrel where the highest thermal gradient has been found to develop, the barrel is heated more intensely as the heaters cycle "off" less frequently.
Within the barrel, a screw rotates, shearing the feed stock and moving it longitudinally through the various heating zones of the barrel causing the feed stock's temperature to rise and equilibriate at the desired level when it reaches the hot or shot end of the barrel. At the hot section of the barrel, the processed material exhibits temperatures generally in the range of 1050.degree.-1100.degree. F. The maximum temperatures subjected to the barrel are in the range of 1140.degree. F. for magnesium processing. As the feed stock is heated into a semisolid state where it develops its thixotropic properties, the interior surface of the barrel correspondingly sees a rise in its temperature. This rise in interior surface temperatures occurs along the entire length of the barrel, including the cold section when its extent is lesser.
Once a sufficient amount of material is accumulated in the hot section of the barrel and the material exhibits its thixotropic properties, the material is injected into a die cavity having a shape conforming to the shape of the desired article of manufacture. Additional feed stock is then introduced into the cold section of the barrel, again lowering the temperature of the interior barrel surface, upon the ejection of the material from the barrel.
As the above discussion demonstrates, the interior surface of the barrel, particularly in the intermediate temperature region of the barrel, experiences a cycling of its temperature during operation of the thixotropic material molding machine. This thermal gradient between the interior and exterior surfaces of the barrel has been seen to be as great as 350.degree. C.
Since the nickel content of the alloy 718 is subject to be corroded by molten magnesium, currently the most commonly used thixotropic material, barrels have been lined with a sleeve or liner of a magnesium resistant material to prevent the magnesium from attacking the alloy 718. Several such materials are Stellite 12 (nominally 30Cr, 8.3W and 1.4C; Stoody-Doloro-Stellite Corp.), PM 0.80 alloy (nominally 0.8C, 27.81 Cr, 4.11W and bal. Co. with 0.66N) and Nb-based alloys (such as Nb-30Ti-20W). Obviously, the coefficients of expansion of the barrel and the liner must be compatible to one another for proper working of the machine.
Because of the significant cycling of the thermal gradient in the barrel, the barrel experiences thermal fatigue and shock. This was found by the present inventors to cause cracking in the barrel and in the barrel liner. Once the barrel liner has become cracked, magnesium can penetrate the liner and attack the barrel. Both the cracking of the barrel and the attacking of the barrel by magnesium were found to have contributed to the premature failure of the above mentioned barrels.
From the above it is evident that there exists a need for an improved barrel construction, particularly for those large thermal mass barrels of large capacity thixotropic material molding machines.
It is therefore a principle object of the present invention to fulfill that need by providing for an improved barrel construction as well as an improved construction for a thixotropic material molding machine itself.
Another object of the present invention is to provide a barrel construction having improved working life under the above operating conditions.
A further object of the present invention is to provide a barrel construction that is not susceptible to thermal fatigue and shock under the above mentioned operation conditions.
It is also an object of this invention to provide a barrel construction which is less expensive than previously known constructions and which incorporates more readily available materials.
Still another object of this invention is to provide a novel method for producing materials exhibiting thixotropic properties.
Also an object of this invention is to optimize the heat transfer and throughput of the thixotropic molding machine.
Another object of this invention is to decrease heat transfer through the nozzle of the machine to the sprue bushing.
Still another object of this invention is to increase heat transfer from the sprue through the sprue bushing.